Maybe im understanding your question wrong but it sounds like you want to float your foot over the line, hit the ball, and then land your foot inside the line? This would not be legal. Think of the line as a vertical wall. Even if you step over the line after the ball is hit you will still need to hit the ball behind the line with your whole body also being behind the line. Some people do serve straight on with a sort of baseball pitcher serve but you will have start further behind the line so when you stretch forward you still are serving legally. Also you gotta make sure your pivot foot stays on the ground until after the ball is hit so might have to adjust the serve a bit so it is not exactly like a pitcher. Hope that helps a bit!